Compression Ratio and Detonation question

[pooder]

Being an avid reader of this forum, it appears that general consensus is that a compression ratio significantly greater than 9.5 will put an unmodified 3.2 motor at increased risk for detonation with the available pump gas. My question is this: do US owners of ROW cars with 10.3 CR experience significant detonation issues, and if not then why not? Thanks in advance,

Peter

[911pcars]

Higher octane fuel available?

[Eagledriver]

The Mahle 10.3 pistons only have about 9.8 actual CR. The factory ECU maps are conservative enough that the engines do ok with premium fuel.

-Andy

[Charles Freeborn]

Yes, you can get high octane (racing) fuel, but it is very expensive. Here in the Portland Area it's nearly $8 per gallon.
The other workaround to high compression set ups is to go twin plug, but that's not a cheap proposition.
-C

[spuggy]

Originally Posted by Charles Freeborn

Yes, you can get high octane (racing) fuel, but it is very expensive. Here in the Portland Area it's nearly $8 per gallon.

And doesn't actually make as much difference as you might think.

At least, that what I found; based on my not-exactly-extensive test of filling up with 100 octane in at PIR and watching the knock sensor on the next few laps My J&S Safeguard paid for itself last week... - I only noted a tiny amount less timing getting pulled with 100 octane, as compared to premium pump gas. I tried a partial fill of 110 octane another time (heck, that's not cheap - and I didn't want the lead to hurt the narrowband sensor), and got much the same results as 100 octane.

I really believe that the CIS ignition curve throws power away almost everywhere by being far more conservative than it needs to be in most places - while actually still being risky in a couple of places given exactly the right (wrong) conditions. I found this with both US and RoW ignition curves, although the RoW was more aggressive - exactly as you'd expect.

Although the 80's Motronic is better, it also has no way to detect (or more importantly, compensate for) conditions/events that will damage the motor, like almost all modern cars do.

I'm biased. I'm a fan of active knock detection/control in general - and a huge fan of the J&S Safeguard in particular. Much simpler/cheaper to add it than than to fix any detonation damage afterwards. Plus, on CIS motors, it allows you to add back some ignition timing for those parts of the map (almost all of it, it seems) where it's too conservative.

[Charles Freeborn]

Originally Posted by spuggy

And doesn't actually make as much difference as you might think.

At least, that what I found; based on my not-exactly-extensive test of filling up with 100 octane in at PIR and watching the knock sensor on the next few laps My J&S Safeguard paid for itself last week... -
I really believe that the CIS ignition curve throws power away almost everywhere by being far more conservative than it needs to be in most places - while actually still being risky in a couple of places given exactly the right (wrong) conditions. Plus, on CIS motors, it allows you to add back some ignition timing for those parts of the map (almost all of it, it seems) where it's too conservative.

Interesting.... Perhaps we can meet one day and you can show me the system. It looks like it could go on not only my current 2.7, but also a 3.2 in the future.
-C

[pooder]

Does anyone have a ROW 3.2 and have you had any problems with detonation using the unleaded premium pump gas available in the US?

[911pcars]

Originally Posted by Charles Freeborn

Yes, you can get high octane (racing) fuel, but it is very expensive. Here in the Portland Area it's nearly $8 per gallon.
The other workaround to high compression set ups is to go twin plug, but that's not a cheap proposition.
-C

I was referring to 93 octane fuel, not racing gas. It's not as readily available as some might think. Some guys in the Corvette forum attempted to compile a list:
93 Octane Gas - Locations Across USA - Corvette Forum

While not quite $8/gal., it's pretty close.

Sherwood

[slimlynn1]

I have an 85 ROW 3.2 running on 93 octane pump gas, so far no problems that I know of. Many track days in the hot humid Northeast weather.

[IROC]

My '76 has a 3.2 out of an '88 that was rebuilt with Mahle 10.3:1 pistons. I run 93 on the street (and race fuel on the track). SteveW chip. Many track days in the southeast also. No issues. I've been flogging this engine for well over 10 years now...

[pooder]

I appreciate the responses. My 87 Carrera is going into the shop this winter and I am considering a 3.4 upgrade w/ higher compression pistons in the attempt to compensate for the power loss I experience where I live (9500ft). However, reliability is a major factor for me and I wanted to feel out real life experience with the "reported" 10.3 CR in the ROW cars. Its probable that at 9500 ft detonation wouldn't be an issue, but it will be driven at lower elevations as well.

[darrin]

pooder -- you might want to consider going with twin plugs while you're at it, as this will allow increased advance without detonation creating more power. You might also want to consider bumping to 3.5, requires a bit of case work, but given the $$ of going to a 3.4 in the first place, it's probably an insignificant increase.

Why not go straight to a 993 3.6 x-plant and get a modern engine management system with active knock sensors? I'm still running a stuck 3.2 down here at 5280 feet and continuing to contemplate when/how to upgrade.

[pooder]

Looked into the 3.6 conversion, and if the 3.6 is rebuilt before installation the price for the conversion gets into the 25-30K range. Didn't want to spent 15K with a used motor with little known about its history, and I understand that all 50K+ mile 993 motors need valve guides.